Tungsten slip casting method



This invention relates to an improved method of forming a tungsten slip casting slurry.

There are many difiiculties in fabricating various types of tungsten articles by conventional metallurgical techniques. To make a crucible or a thin-walled tube of tungsten, one method which may be essayed is to form the article from sheet stock on a mandrel and butt weld the joint. Here the difliculties encountered are the brittleness of the material, which makes forming on a mandrel diificult, and the absence of a well developed technique for welding tungsten. The conventional powder metallurgy technique of dry pressing and sintering the thin tube is severely limited with respect to minimum wall thickness in that the strains induced by pressing may result in a warped final product. On the other hand, a shape such as a thick-walled blank suitable for deep drawing into a long, thin-walled capsule is difiicult to form by pressing, punching or forging without numerous defects. The thick blank is diflicult to prepare by machining because of the extreme intransigence of tungsten to machining operations.

It is known that prior workers have investigated various aqueous slurn'es or suspensions for slip casting. Such compositions were found to be inadequate in that they do not consistently produce high density crack-free castings. Either the green castings are cracked, crack during sintering, or are not sufliciently close to maximum percentage of theoretical density after final sintering.

A good tungsten slip must permit wall formation uniformly from top to bottom of mold, i.e., without setting of the heavy tungsten particles. The vehicle for holding the metal powder in suspension must not minimize the metal powder to vehicle ratio and must have the other properties of a casting slip-nonreactive, good drainage characteristic, reasonably safe to use, etc.

An improvement over the prior art described above was found when organic slurries or suspensions were utilized for the slip casting of tungsten. However, the use of these organic vehicles has basic drawbacks. For instance, a single organic material satisfying all the desired characteristics has not been found. In attempting to use organic vehicles, it has been found necessary to use at least two, and preferably three or four substances other than the metal powder itself (US. Patent No. 3,052,532). The required two substances are the basic vehicle itself furnishing the necessary fluidity and drainage and the other a thickening agent which aids in holding the metal particles in suspension during the casting process. It has also been found advantageous to use a minor proportion of wetting agent, i.e., materials that promote wetting of the metal particles by the other constituents of the slip. Another useful additive is a minor fraction of one or more materials that act as deflocculants, dispersing the metal particles without conglomeration, and/ or thinning the slip to promote better drainage. It is seen that the use of at least two and preferably four complex (and expensive) organic chemical vehicles constitutes a drawback in such a rocedure. Furthermore, although the fired densities are in the range of 93 to 97% theoretical density, the accompanying firing shrinkage amounts to 30 to 45%.

The present invention permits the use of a simple aqueous vehicle rather than complex organic chemical States Patent 3,322,536 Patented May 30, 1967 vehicles. The amounts of aqueous vehicle required is minimal compared to that required when complex organic chemical vehicles are utilized. In addition, a high fired density (up to 97% theoretical density) is obtained, as well as low firing shrinkage 17 to 27% This advance in the art has been accomplished by coating the individual tungsten particles. The coating may be of either carbon or oxide. This coating permits easy deflocculation in an aqueous vehicle otherwise unobtainable with uncoated tungsten powder.

The use of an oxide coating possesses certain advantages over the use of carbon coating. For example, when working with certain pure metals, the excessive residual carbon content of the ware made using carbon coatings may be objectionable. Furthermore, a minimal amount of organic deflocculant is necessary when forming a casting slip with carbon coated tungsten powder. The use of an oxide coat obviates both of the above difiiculties.

Accordingly. it is an object of this invention to provide a method of treating tungsten powder so that it may be easily defiocculated and less reactive with an aqueous vehicle.

It is a further object of this invention to provide an improved method of tungsten slip casting so that finished articles will exhibit a fired density of 94 to 97% theoretical density with an accompanying firing shrinkage o 17 to 27%.

OXIDE COAT PROCEDURE Tungsten powder of varying sizes (0.7 to 0.9 micron average size) was added to a 0.4 gallon high-alumina porcelain ball mill together with 7 inch diameter WC balls. Suitable quantities are, for example, 1200 grams tungsten powder added to 4400 to 4600 grams of WC balls. The mill is rotated at 55 rpm. for specified times, opened to the atmosphere, rescaled, and rotated again. This procedure is repeated until no additional oxidation. is apparent (noticeable vacuum in the mill on opening). A typical mill opening and closing cycle is:

Mill two hours and open Close mill and run 30 minutes before opening Repeat procedure at 30-minute intervals until total milling time elapsed is 3 /2 hours r Now open at 15-minute intervals until 5 hours total milling time has elapsed.

At each opening of the mill, the mill interior is scraped down. Total milling time has been varied from two hours to seven hours and the optimum found to be five hours. After dry milling, the powder is screened through a 325 mesh US. Standard screen to remove agglomerates.

In order to form the slip, all that is necessary is to add a small quantity of water, for example, 6 to 7 cc. of distilled water per 100 grams of coated powder. When the mixture of water and powder is thoroughly stirred, then cast in conventi-onal plaster molds, it yields inch wall thickness in one minute. The cast piece releases well from the mold and trims easily with no cracking or tearing.

The cast ware is fired to 1800 C. with 15-hour soak at peak temperature resulting in fired density of 95 to 97% with an average fired shrinkage of 18 to 20% The casting slip exhibits a good shelf life, being useable after several hours storage when thinned with one or more drops/ 100 cc. slip of a saturated solution of tetrasodium pyrophosphate.

CARBON COAT PROCEDURE Example] 600 gm. of 0.7 average particle size W powder and 9 gm. of powdered polyethylene were mechanically mixed r 3 and fired to 625 C. in H for one hour. The material was then screened to 325 mesh (U.S. Standard Screen Series) to remove agglomerates and a casting slip compounded for it as follows:

W powder gm 100 Distilled H O cc 9 40% aqueous solution of a neutral sodium salt of aryl sulfonic acid (e.g., TAMOL N made by Rohrn & Haas)a neutral sodium salt of a condensed naphthalene sulfonic acid is preferred drops 4 The result-ing slip is cast in conventional plaster molds, draining well and yielding inch wall thickness in one minute. The resulting crucibles were fired, after drying, in an H atmosphere furnace to 1825 C. exhibiting a fired density of 91.0% T.D. with accompanying firing shrinkage of 26 to 27%.

Example 2 W powder gm 100 Distilled H O a cc 7 40% aqueous solution of a neutral sodium salt of aryl sulfonic acid drops 5 The resulting slip was cast in the manner described above yielding inch thick walls in 1 to 2 minutes. Crucibles thus produced were fired to 1825 C. in an H atmosphere exhibiting a fired density of 94 to 95% TD. with accompanyiug firing shrinkage of 21 to 23%.

Example 3 800 gm. of 0.9 average particle size W powder was dry-milled together with 6 gm. flake graphite in a 0.4 gallon high density porcelain mill jar with WC balls for eight hours. After screening to 325 mesh, this material was compounded into a slip as follows:

W powder gm 100 Distilled H O cc 6.5-7 40% aqueous solution of a neutral sodium salt of aryl sulfonic acid drops 5 Cruci bles fabricated from the W powder processed in this manner compare favorably with those produced by the former methods described. Carbon black was substituted for graphite powder in this approach and yielded identical results.

Clearly neither the method of firing (vacuum or hydrogen) nor the source of carbon can be considered limitations. Other techniques for forming the carbon layer may be used, e. g., pyrolytic coating, fluidized bed, etc.

Treating 2.5 average particle size W powder in a similar manner (oxide or carbon coat) and combining with approximately 70 to 75% of the 0.9a material yields graded particle size (approximating Fullers curve) which, in turn, yields fired Ware with shrinkage as low as 17% with no sacrifice in density.

The above examples have been set forth as illustrative of the present invention and said invention is not restricted to the above specific examples. For example, it is clear that slurries produced in the manner described above can be used to provide an adherent tungsten coating on ceramic crucibles (e.g., A1 0 for use as electrically conductive coatings for induction heating. It is also clear that such a slurry may be brushed and fired on a ceramic, such as A1 0 and some cermets, such as MoUO resulting in a metallized layer for use in fabricating high temperature ceramic-metal seals. A tungsten coating on the cermet Mo-UO is particularly advantageous since the coating inhibits U0 migration and loss at elevated temperatures. Consequently, we wish only to be restricted by the following appended claims.

What is claimed is:

1. The method of forming a slip casting slurry which comprises the steps of carbonizing a tungsten powder and adding to said tungsten powder, an aqueous solution of a neutral sodium salt of aryl sulfonic acid.

2. The method of claim 1 wherein to every 100 grams of said tungsten powder are added 69 cc. of distilled water and 4-5 drops of a 40% aqueous solution of a neutral sodium salt of aryl sulfonic acid.

3. The method of claim 1 wherein the tungsten is carbonized by dry milling the tungsten powder with a material selected from the class consisting of charcoal and carbon black.

4. The method of claim 1 wherein the tungsten powder is carbonized by mixing and firing powdered polyethylene with the tungsten powder at elevated temperatures in a hydrogen atmosphere.

5. The method of claim 1 wherein the tungsten powder is carbonized by milling tungsten inv a polyethylene lined ball mill with WC balls and CCl, and subsequently drying and firing in a hydrogen atmosphere.

6. The method of claim 1. wherein the tungsten powder is screened to 325 mesh prior to mixing with the aqueous solution.

7. The method wherein the slurry prepared in claim 1 is cast in plaster molds, dried, and the resulting article is fired in a hydrogen atmosphere.

8. The method wherein the slurry prepared in claim 1 is applied to a ceramic surface, dried, and then fired to form a ceramic-metal seal.

9. The method as in claim 8 wherein the ceramic surface comprises A1 0 10. The method of forming a slip casting slurry which comprises the steps of substantially completely carbonizing a tungsten powder and adding to said tungsten powder an aqueous solution of a neutral sodium salt of aryl sulfonic acid.

11. The method of forming a slip casting slurry which consists essentially of the steps of carbonizing a tungsten powder and adding to said tungsten powder an aqueous solution of a neutral sodium salt of aryl sulfonic acid.

12. The method of forming and using a slip casting slurry which comprises the steps of oxidizing the surface of a tungsten powder and adding water to said tungsten powder, applying the resulting slurry to a ceramic surface, drying, and then firing to form a ceramic-metal seal.

13. The method of forming and using a slip casting slurry which comprises the steps of oxidizing the surface of a tungsten powder and adding water to said tungsten powder, applying the resulting slurry to a M0-UO surface, drying, and then firing to form a cermet-metal seal.

References Cited UNITED STATES PATENTS 2,082,354 6/1937 Reichmann 201 2,242,254 5/1941 Mansfield 14813.1 3,115,698 12/1963 St. Pierre 75211 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. I, Interscience Publishers, Inc., New York, 1949, page 237, TN 695 G6.

Metals Handbook, 8th edition, volume I, American Society for Metals, Novelty, Ohio, 1961, page 7, TA472 A3.

CARL D. QUARFORTH, Primary Examiner.

REUBEN EPSTEIN, Examiner.

L. D. ROSDOL, R. L. GRUDZIECKI,

Assistant Examiners. 

1. THE METHOD OF FORMING THE SLIP CASTING SLURRY WHICH COMPRISES THE STEPS OF CARBONIZING A TUNGSTEN POWDER AND ADDING TO SAID TUNGSTEN POWDER, AN AQUEOUS SOLUTION OF A NEUTRAL SODIUM SALT OF ARYL SULFONIC ACID.
 13. THE METHOD OF FORMING AND USING SLIP CASTING SLURRY WHICH COMPRISES THE STEPS OF OXIDIZING THE SURFACE OF A TUNGSTEN POWDER AND ADDING TO WATER TO SAID TUNGSTEN POWDER, APPLYING THE RESULTING SLURRY TO A MO-UO2 SURFACE, DRYING, AND THEN FIRING TO FORM A CEMENT-METAL SEAL. 